Legal Question in Criminal Law in Tennessee

She should check with her attorney, or she should ask the District Attorney, because "the underlying charge" is not defined in Tennessee. It appears to be shorthand used by the District Attorney in your district.

IF it was in my district, it would mean: "I will agree that the Defendant is to serve 30% of the original sentence in jail" to close out the case.

In other words: If she was originally convicted of Driving on a Revoked License, was sentenced to six months - 180 days - of probation, and then violated probation, the 30% would mean:

"I will agree, (subject to the court's approval, of course) that she will serve 30 percent of the original sentence = 54 days in jail as punishment for the violation. Then the case will be over."

"Or, she can have a hearing in front of the judge, who might order her to serve the full 180 days in jail for the violation of her probation."